Semiconductor integrated circuits are typically fabricated by a layering process in which several layers of material are fabricated on a surface of a wafer. This fabrication process typically requires subsequent layers to be fabricated upon a smooth, planar surface of a previous layer. However, the surface topography of layers may be uneven due to an uneven topography associated with an underlying layer. As a result, a layer may need to be polished in order to present a smooth, planar surface to a subsequent processing step. For example, an insulator layer may need to be polished prior to formation of a conductor layer or pattern on an outer surface thereof.
In general, a semiconductor wafer may be polished to remove high topography and surface defects such as scratches, roughness, or embedded particles of dirt or dust. The polishing process typically is accomplished with a polishing system that includes a wafer carrier and a polishing table between which the semiconductor wafer is positioned. The wafer carrier and the table are moved relative to each other thereby causing material to be removed from the surface of the wafer. The polishing process may also involve the introduction of a chemical slurry to facilitate higher removal rates, along with the selective removal of materials fabricated on the semiconductor wafer. This polishing process is generally referred to as chemical-mechanical planarization or chemical-mechanical polishing (CMP). The chemical slurry is generally an aqueous acidic or basic solution having a number of abrasive particles, such as silica (SiO2), alumina (Al2O3), or ceria (Ce2O3) particles, suspended therein.
The size of the abrasive particles of the chemical polishing slurry is desirably within a range of, for example, 0.03–0.4 microns. However, the polishing slurry can often include larger particles which can lead to scratching of the wafer or other types of manufacturing problems. The large particles may be aggregates (i.e., multiple smaller particles chemically bonded to one another), agglomerates (i.e., “clumps” of particles or aggregates), or gels. Occasionally, the polishing slurry may also contain foreign particles which require removal from the slurry.
The conventional approach to remove large particles from a polishing slurry includes the use of a series of progressively tighter depth filters. A first depth filter having a 10 micron cutoff is followed by a number of progressively tighter filters down to a final depth filter located proximate the polishing tool having a cutoff in the range of 1.0–1.5 microns.
The use of such depth filters has a number of drawbacks associated therewith. For example, existing depth filters are not “absolute” filters—they only remove a portion of the particles greater than their micron cutoff. As such, a number of large particles escape the filters. Moreover, use of such filters is expensive since depth filters capture the particles on the filter surface (or inside the filter path), and, as a result, require replacement on a frequent basis.